A single laser beam, in particular a beam from a single-mode laser, typically has an about Gaussian intensity distribution (profile) in at least one transverse axis of the beam. Certain laser material processing applications, however, require a relatively flat intensity profile in at least one axis. Uniformity in one axis is usually adequate where the beam is formed having a line or bar cross-section shape. In those applications in which a beam must have a round or square cross-section shape, uniformity in transverse axes perpendicular to each other (X and Y or sagittal and tangential) is required. A beam having such a uniform cross-section is usually referred to as a flat-topped beam or a “top-hat” beam.
Laser material processing applications can include, for example, drilling holes in printed circuit boards. In this case, uniformity in both transverse axes is required to provide holes with minimized sidewall taper. Other applications include processing of glass, ceramics, or silicon wafers, which can include processes such as annealing, cutting and fusing. Many of these applications require a beam having a relatively high power, for example, about 100 Watts (W) or more. A gas-discharge laser, such as a carbon dioxide (CO2) laser, is usually preferred for these applications. A gas discharge laser having multimode output is often required to efficiently achieve the required power.
One common approach to providing a flat-topped beam is to pass a beam having a Guassian intensity distribution (a Gaussian beam) through an aperture smaller than the beam cross-section such that only the center portion of the Gaussian beam is transmitted. This provides a relatively poor approximation of a flat topped beam and a significant amount of power in the original laser beam is lost due to the discarding of the portion of the original beam not transmitted through the aperture.
Another approach is to utilize a homogenizing device including diffractive optics or one or more pairs of cylindrical lens arrays. A beam from a high-power, gas-discharge laser, such as a CO2 laser, tends to have varying multimode output over time and numerous small discharge “hot spots” within the beam. This causes numerous amplitude variations (noise) in the output beam. When these amplitude variations have periods that are comparable to or longer than the thermal time constant of the material to be processed, unacceptable variations occur in the process.
Yet another approach to providing a high-power flat topped laser beam is disclosed in U.S. Pat. No. 7,199,330 (“the '330 patent”), which issued on Apr. 3, 2007, to DeMaria et al. and which is hereby incorporated herein by reference. As disclosed in the '330 patent, a plurality of Gaussian laser beams from a corresponding plurality of CO2 lasers and having about the same cross-section size are partially overlapped to provide a combined beam having an about uniform cross-section. In order to avoid low frequency amplitude variations in the combined beam due to interference effects, electronic circuitry is used to frequency stabilize each laser at stable single frequencies different from each other by a few megahertz (MHz). This difference between the single frequencies is selected to be sufficient such that any interference beat frequencies that occur are sufficiently high that resulting amplitude variations occur over a time period much shorter than the thermal response time of material being processed.